Front projection screens for displaying images projected by a projector include white matrix screens, chemical etching screens, and high-luminance bead screens. When an image is projected on such a screen by a projector in a well-lighted room, the image cannot be displayed clearly due to the reduction of contrast or the loss of color balance. Therefore, room lights of the room must be turned off to darken the room. When images are projected on a high-luminance bead screen by a high-luminance projector, a person is made to feel pains in the eyes by the glare of beads due to light regression when the person watches images displayed on the high-luminance bead screen for a long time. The term “light regression” signifies return of imaging light in a direction opposite to the traveling direction of projected imaging light.
A composite screen proposed in JP-A No. 206620/2000 is capable of displaying an image of an excellent color balance without reducing the contrast of the image even in a light environment without requiring room lights to be turned off. This prior art composite screen is constructed by disposing a generally transparent, front shading sheet near a surface of a conventional front projection screen on the viewing side. The front shading sheet is provided with a plurality of horizontal, parallel, minute ridges having a triangular cross section in its front surface, and a shading layer is formed on one of the two sides of each ridge having a triangular cross section.
FIGS. 14 and 15 show a composite screen of the same construction as the screen disclosed in JP-A No. 206620/2000, formed by disposing a glass bead front projection screen analogous with a front projection screen embodying the present invention behind a front shading sheet. As shown in FIGS. 14 and 15, a front projection screen 33 is formed by bonding transparent glass beads 32 to the front surface of an opaque sheet 30 with a white, reflective adhesive layer 31. A front shading sheet 35 of a transparent material is disposed on the viewing side A of the front projection screen 33 such that an air gap 36 is formed between the front surface of the front projection screen 33 and the front shading sheet 35 to enable viewing images of an excellent color balance without reducing the contrast of images even in a light environment without turning off room lights. The front shading sheet 35 is provided with a plurality of horizontal, minute ridges 38 having a triangular cross section in its front surface, and a black shading layer 39 is formed on an upper side surface of each minute ridge 38. The front shading sheet 35 enables viewing images of an excellent color balance without reducing the contrast of the images owing to effects explained in JP-A No. 206620/2000 and to be explained later.
No problem arises when a person views an image displayed on the composite screen shown in FIGS. 14 and 15 from a position in front of a central part of the composite screen. However, if a person views obliquely an image displayed on the composite screen from a position at a viewing angle greater than a certain horizontal viewing angle of, for example, 140°, a peripheral part of the image displayed on the composite screen darkens, a part of the image disappears, and the viewing angle narrows.
Reasons for such problems will be described with reference to FIGS. 14 and 15. Referring to FIG. 15, imaging light rays R1 projected by a projector P disposed on the viewing side A travel straight through the front shading sheet 35, penetrate into the transparent glass bead 32, are reflected by the back surface of the transparent glass bead 32. The reflected imaging light rays emerge from the glass bead 32, travel through the air gap 36, enter the front shading sheet 35, are refracted by the front shading sheet 35, and emerge forward from the front shading sheet 35 in the directions of the arrows R2, R3, R4, R1, R5, R6 and R7. Thus, persons in a region between the arrows R2 and R7 are able to see an image formed by the imaging light rays projected by the projector P.
The imaging light rays represented by the arrows R2, R3, R4, R1, R5, R6 and R7 include a large quantity of light including vertical components in addition to horizontal components. When such imaging light rays are reflected by the back surface of the transparent glass bead 32, reflected light rays including both vertical components and horizontal components penetrate into the front shading sheet 35. Some of the reflected light rays incident on the front shading sheet 35 fall on the lower surface of the black shading layer 39 as shown in FIG. 14. The reflected light rays fallen on the lower surface of the black shading layer 39 are reflected totally on to the rear surface 41 of the front shading sheet 35 as indicated by the arrow Rt. The reflected rays are reflected again by the rear surface 41 and travel forward in the direction of the arrow Rt from the front shading sheet 35. Most of the reflected light rays fallen on the rear surface 41 at incident angles smaller than the critical angle travel rearward through the rear surface 41, while the reflected light rays fallen on the rear surface 41 at incident angles greater than the critical angle are totally reflected by the rear surface 41. Therefore, all the light rays Rt that emerge from parts of the front shading sheet 35 horizontally apart from the center of the front shading sheet 35 are reflected in a total reflection mode by the rear surface 41.
The light rays Rt reflected by the rear surface 41 in a total reflection mode are dark light rays reflected by the lower surface of the black shading layer 39. Those dark light rays make an image formed by the light regressed from the transparent glass bead 32 turbid. It will be understood from the foregoing explanation that the dark light rays Rt emerge from parts horizontally apart from the center of the front shading sheet 35 because the light rays Rt fall on the rear surface 41 of the front shading sheet 35 at incident angles greater than the critical angle. Consequently, horizontal end parts of the image look dark to persons viewing the image from positions corresponding to horizontal end parts of the front shading sheet 35. The light rays Rt include both vertical components and horizontal components as a matter of course, the fact that the parts of the image displayed on horizontal end parts of the front shading sheet 35 look dark signifies that the viewing angle is small because it is possible that the viewing position changes greatly in horizontal directions while the viewing position does not change greatly in vertical directions.
Accordingly, it is an object of the present invention to solve problems that darkens part of a front projection screen, and to provide a front projection screen having a large viewing angle, not reflecting an environmental image even in a light environment, and capable of displaying a high-definition image having a high black level and a high contrast.